Dual passage cold trap



FOREVACUUM May 30, 1967 c E; MlNER ET AL 3,322,330

DUAL PASSAGE COLD TRAP Filed Jan. 26, 1965 PUMP INVENTORS CLAIRE E. MINER Y RICHARD M. REIMERS ATTORNEY Patented May 30, 1967 3,322,330 DUAL PASSAGE Ctl Ll) TRAP Claire E. Miner, Wa nut Cree-la, and Richard M. Reimers,

Eerkeley, Calih, assignnrs to the United dtatcs of America as represented by the United States Atomic Energy Commission Fiiad Jan. 26, 1965, Ser. No. 428,261 2 Claims. (C2. ZIifi-dtflll) ABSTRACT OF THE DiSCLGfiURE This invention relates to a vacuum system cold trap between a diffusion pump and a chamber being evacuated and provides refrigerated surfaces for condensing backstreaming pumping fluid and other vacuum contaminants. The invention provides dual gas channels having condensation surfaces cooled by a single volume of refrigerant whereby the pump intake is connected to the chamber through a first channel and the pump outlet is connected to the forevacuum system through an insulated second channel. The refrigeration of the pump outlet increases the pumping efficiency and degree of vacuum obtainable.

This invention relates to cold traps for use in vacuum pumping systems and more specifically to the type of cold trap which is disposed between a diffusion pump and the chamber to be evacuated and which contains refrigerated surfaces for condensing backstreaming pumping fluid vapor and other vacuum contaminants. The invention described herein was made in the course of, or under, Contract W7405eng-48 with the US. Atomic Energy Commission.

Liquid nitrogen filled traps are used with diffusion pumps in vacuum systems to increase the effectiveness of the pump, to reduce back-streaming and to reduce creep of the pumping fluid which condenses on the wall of the pump. Commonly the trap consists of a passageway connecting the input of the pump to a vessel to be evacuated. Adjacent the top of the pump a section of the passage is in bellows form. The passage bove the bellows is surrounded by a container for liquid nitrogen. A metal baflle, generally of the chevron type, extends across the passage so there is no line of sight opening therethrough. The battle is cooled by the liquid nitrogen through contact with the sides of the passage. The upper part of the bellows section, being in contact with liquid nitrogen cooled surfaces and therefore at a low temperature, acts to stop any creepage that may occur.

Heretofore, the output of a diffusion pump has generally been connected directly to either the forevac line or to a booster pump. The connection thus established and the pumps used determine the back pressure against which the diffusion pump operates.

In the present invention the middle section of the cold trap passage is enlarged and a tubular connection at an angle thereto connects with the vessel to be evacuated. In a preferred form, a container holding liquid nitrogen projects into the enlarged portion of the passage so that gas molecules must pass therearound and thus the container serves as the baffle. A second housing surrounds the liquid nitrogen container and the enlarged portion of the passage in spaced apart relation thereto to form a secand vacuum channel. The inner wall of the housing is heat insulated and inlet and outlet tribulations connect thereto. The output of the diffusion pump is connected to the forevac line or to a booster diffusion pump through the second channel so that the single cold trap is operative on both the inlet and outlet of the pump.

By providing liquid nitrogen trapping in the outlet line,

as well as the inlet of the diffusion pump, the invention greatly improves pumping efficiency by increasing both the amount of gas that the pumps in the forevac line will handle and the amount the diffusion pump will handle by reducing the back pressure the pump works against. Also the vacuum chamber and the insulation both of which surround the liquid nitrogen container and passage permits obtaining lower temperatures, economizes on the amount of liquid nitrogen required even for a single trap, decreases the possibilities of air leakage into the high vacuum portion of the system and permits refrigerated trapping of two parts of the system with a single container of liquid nitrogen.

It is an object of this invention to provide a more efficient cold battle for use with diffusion pumps.

It is an object of the present invention to provide means for attaining a higher vacuum with a diffusion pump than could otherwise be obtained.

Another object of the present invention is to reduce the back pressure against which a diffusion pump is required to work.

Still another object of this invention is to provide an economical and compact cold trap capable of providing a refrigerated baffling within a plurality of flow passages.

A further object of the present invention is to provide cold baffles for both input and output connections of a diffusion pump in one unit and having a single container of refrigerant.

The invention with further objects and advantages thereof will be better understood by reference to the following specification and accompanying drawing, of which:

FIGURE 1 is an elevation view showing a cold trap and illustrating the manner in which the trap is connected to a vacuum chamber, diffusion pump forevacuum system, the trap being partly cutaway to show interior elements thereof,

FIGURE 2 is a cross-section view of the trap taken along line 22 of FIGURE 1, and

FIGURE 3 is a cross-sectional view of the circled portions of FIGURE 1 showing an enlarged cross-section of a portion of the outer wall of the trap and its insulating lining.

Referring now to the drawing and more particularly to FEGURE l, the cold trap 9 is typically disposed between a diffusion pump 10 and the vessel 11 which is to be evacuated, the trap being at the top or inlet end of the pump. The trap 9 has a first passageway 12 for connecting the intake of pump 10 to the vessel 11.

The central portion of the passage 12 is formed between a first cylindrical vessel 13 and a second cylindrical vessel 14 of lesser diameter, containing liquid nitrogen 16, which is sealed into the top of the first vessel in coaxial alignment therewith. Vessel 14 protrudes from the top of first vessel 13 and extends down into the first vessel to near the bottom thereof and in this manner provides space for the passage 12 between the bottoms and the sides of the first and second vessels. A tubulation 17 which connects with an opening in the bottom of the first vessel 13, in coaxial alignment therewith, extends passage 12 downwardly. This tubulation 17 has a first stainless steel bellows section 18 adjacent the bottom of vessel 13 to which it is sealed and a tubular section 19 sealed to the other end of the bellows section. Tubular section 19 has a flange 21 at the end of the tubular section remote from the bellows section thus providing for connection to the input to diffusion pump 10. The passage 12 is also extended at its other end by a tubulation 23 which has a bellows section 24 sealed into the side wall of first vessel 13 and has a tubular section 26 sealed to the other end of the bellows section and terminated at a L) flange 27 which thus provides for connection to the vessel 11 which is to be evacuated.

The two bellows sections 18 and 24 are both made of thin stainless steel so that these sections will conduct very little heat to the areas cooled by liquid nitrogen. The bellows sections 18 and 24 also act to relieve stresses by taking up differences in expansion and contraction of various parts. The bellows section 18 which is in the passage directly connecting to the diffusion pump 19 also operates to reduce creepage of condensed fuel oil from the inner walls of the diffusion pump int-o the vessel 11 which is being evacuated since the inner end thereof is at liquid nitrogen temperature.

To form a second refrigerated channel for connecting the outlet of pump to the forevacuum system through the same cold trap 9, a cylindrical outer housing 29 surrounds the first vessel 13 and the smaller vessel 14 in spaced-apart relationship thereto. The previously described tubulations 17 and 23 transpierce housing 29 with the bellows sections 18 and 24 of the tubulations being sealed thereto. The inner walls of housing 29 are lined with thermal insulation 31, shown in more detail in FIG- URE 3. Insulation 31 consists in this embodiment of nine thin sheets 32 of stainless steel each provided with dimples 33 at regular intervals to provide for a narrow spacing between each pair of sheets 32 and between the sheets 32 and the housing 29.

An outlet conduit 34 connects into the side of housing 29 near the bottom thereof and is provided with a flange 36 at the end for connection to a booster pump or other forevacuum systems in the conventional manner. An inlet conduit 37 connects with the side of housing 29 near the top thereof and is provided with a flange 38 at the end for connection to the output 39 of the diffusion pump 10.

A fill and vent tubulation 41 is provided for the liquid nitrogen container 14 and consists of an inner tube 42 of thin stainless steel extending upwardly from the top of vessel 14 and an outer tube 43 coaxial therewith and spaced apart therefrom, the outer tube extending upwardly from housing 29. Both inner tube 42 and outer tube 43 are sealed to a flange 44 at the upper ends thereof. The inner tube 42 is thus surrounded by a vacuum and being of thin stainless steel will conduct very little heat to the areas cooled by the liquid nitrogen.

In operation, the gradual evaporation of the liquid nitrogen 16 within inner vessel 14, through vent 41, maintains the inner vessel 14 and intermediate vessel 13 at a very low temperature. As the surfaces of the vessels 13 and 14 form portions of the passages between vacuum chamber 11 and pump 10 and between the pump and the forevacuum system, a cold trapping action is provided in each thereof. Condensible fluids, such as pumping fluid vapor from the pump 10 which attempt to move through the passages are condensed and frozen upon contact with the refrigerated surfaces.

The invention has several advantages over systems now used. A unique advantage results from the cold baffle that is provided in the output from the diffusion pump. This provision reduces the back pressure against which the pump works and thereby increases the final degree of vacuum that may be obtained by several orders of magnitude. A further advantage of the double passage structure is the better insulation of the inner passage 12 due to the presence of the second refrigerated passage therearound. This added insulation and better cooling of the inner passage improves the efiiciency of the diffusion pump and contributes to the increased degree of vacuum that may be obtained. The materials used in this cold trap, as previously described, are such that it may be baked out at 450 degrees centigrade to free and remove adsorbed contaminants.

Although this invention has been disclosed with respect to a single embodiment it will be evident to those skilled in the art that many variations are possible within the spirit and scope of the invention. Therefore it is not intended to limit the invention except as defined by the following claims.

What is claimed is:

1. A cold trap for use with a diffusion pump in a vacuum system comprising, in combination, a first tubulation having means for connecting one end thereof to the input of the diffusion pump and the other end to a vessel to be evacuated and providing a non-linear gas passage therebetween, a container holding liquid gas disposed to cool the central portion of said first tubulation, said container having an opening for filling and venting thereof, and a housing surrounding said container and the central portion of said tabulation and having a heat insulating lining covering all the interior surfaces thereof, said housing further having conduits sealingly connected thereto for connecting said housing with the output of said diffusion pump and with the input of a forevacuum pump whereby gas molecules contact a refrigerated surface both before entering and after leaving said diffusion pump.

2. A cold trap for use with a diffusion pump in a vacuum system comprising, in combination, a first tubulation having means for connecting one end thereof to the input of the diffusion pump and the other end to a vessel to be evacuated and providing a non-linear passage therebetween, said first tubulation having a cylindrical bellows section at each end of a central portion thereof to retard the flow of heat therealong thus insulating the central part of said tabulation from heat conduction from said two end connections, a container holding liquid gas disposed to cool the central portion of said first tubulation, said container having an opening for the filling and venting thereof, and a housing surrounding said container and the central portion of said tubulation and having conduits sealingly connected thereto for connecting said housing with the output of said diffusion pump and with the input of a forevacuum pump whereby gas molecules contact a refrigerated surface both before entering and after leaving said diffusion pump.

References Cited UNITED STATES PATENTS 1,583,473 5/1926 Klopsteg et al 230101 2,931,561 5/1926 Hiesinger 230--101 3,144,756 8/1964 Arnold et al. 230-401 3,175,373 3/1965 Holkeboer et al. 62268 3,256,676 6/1966 Blauth 230-101 3,258,196 6/1966 Knox et al. 230-101 DONLEY J. STOCKING, Primary Examiner.

W. L. FREEH, Assistant Examiner 

1. A COLD TRAP FOR USE WITH A DIFFUSION PUMP IN A VACUUM SYSTEM COMPRISING, IN COMBINATION, A FIRST TUBULATION HAVING MEANS FOR CONNECTING ONE END THEREOF TO THE INPUT OF THE DIFFUSION PUMP AND THE OTHER END TO A VESSEL TO BE EVACUATED AND PROVIDING A NON-LINEAR GAS PASSAGE THEREBETWEEN, A CONTAINER HOLDING LIQUID GAS DISPOSED TO COOL THE CENTRAL PORTION OF SAID FIRST TUBULATION, SAID CONTAINER HAVING AN OPENING FOR FILLING AND VENTING THEREOF, AND A HOUSING SURROUNDING SAID CONTAINER AND THE CENTRAL PORTION OF SAID TUBULATION AND HAVING A HEAT INSULATING LINING COVERING ALL THE INTERIOR SURFACES THEREOF, SAID HOUSING FURTHER HAVING CONDUITS SEALINGLY CONNECTED THERETO FOR CONNECTING SAID HOUSING WITH THE OUTPUT OF SAID DIFFUSION PUMP AND WITH THE INPUT OF FOREVACUUM PUMP WHEREBY GAS MOLECULES CONTACT A REFRIGERATED SURFACE BOTH BEFORE ENTERING AND AFTER LEAVING SAID DIFFUSION PUMP. 